Optical fibers provide significantly higher data rates than electronic paths. However, effective utilization of the greater bandwidth inherent in optical signal paths requires optical cross-connect switches. In a typical telecommunications environment, switching of signals between optical fibers utilizes an electrical cross-connect switch. The optical signals are first converted to electrical signals. After the electrical signals have been switched, the signals are again converted back to optical signals that are transmitted via the optical fibers. To achieve high throughput, the electrical cross-connect switches utilize highly parallel, and highly costly, switching arrangements. However, even with such parallel architectures, the cross-connect switches remain a bottleneck.
A number of optical cross-connect switches have been proposed; however, none of these have successfully filled the need for an inexpensive, reliable, optical cross-connect switch. One class of optical cross-connects depends on wavelength division multiplexing (WDM) to affect the switching. However, this type of system requires the optical signals being switched to have different wavelengths. In systems where the light signals are all at the same wavelength, this type of system requires the signals to be converted to the desired wavelength, switched, and then be re-converted to the original wavelength. This conversion process complicates the system and increases the cost.
A second type of optical cross-connect utilizes total internal reflection (TIR) switching elements. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The magnitude of the change of direction depends on the difference in the index of refraction of the two regions. To obtain a large change in direction, the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that differs markedly from that of the waveguide.
Prior art TIR elements that provide a large change in index of refraction operate by mechanically changing the material behind the boundary, and hence, have relatively slow switching speeds. In addition, mechanical devices have reliability problems. For example, U.S. Pat. No. 5,204,921, Kanai, et al describes an optical cross-connect based on an array of crosspoints in a waveguide. A groove at each crosspoint, may be switched "on" or "off," depending upon whether the groove is filled with an index-matching oil. The index-matching oil has a refractive index close to that of the waveguides. An optical signal transmitted through a waveguide is transmitted through the crosspoint when the groove is filled with the matching oil, but the signal changes its direction at the crosspoint through total internal reflection when the groove is empty. To change the cross-point switching arrangement, grooves must be filled or emptied. In the system taught in this patent, a "robot" fills and empties the grooves. A faster version of this type of TIR element is taught in U.S. Pat. No. 5,699,462 which utilizes thermal activation to displace liquid from a gap at the intersection of a first optical waveguide and a second optical waveguide. However, the switching speed of this device is still relatively slow, and hence the device is limited to applications in which switching speeds of tens of milliseconds are acceptable.
Prior art TIR elements with very fast switching times are also known. These elements alter the index of refraction of the material behind the boundary by applying an electric field to a material whose index of refraction is a function of the electric field. For example, U.S. Pat. No. 5,078,478 describes a TIR element in which the waveguide is constructed in a ferroelectric material. The index of refraction of the ferroelectric material along a boundary within the waveguide is altered by applying an electric field across a portion of the waveguide. While this type of device switches in nanoseconds, the change in index of refraction is very small, and hence, the direction of the light can only be altered by a few degrees. Deflections of this magnitude complicate the design of a cross-point array, and hence, commercially viable cross-connects based on this technology have not been forthcoming.
Broadly, it is the object of the present invention to provide an improved optical cross-connect switch.
It is a further object of the present invention to provide an optical cross-connect switch whose state may be switched faster than mechanically based systems.
It is a still further object of the present invention to provide an optical cross-connect switch that provides large angles of deflection.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.